1. Field of the Invention
The present invention relates to a radio frequency power amplifier which is used to transmit and receive a radio frequency signal. More particularly, the present invention relates to a low-noise and high-efficiency radio frequency power amplifier comprising a bipolar transistor.
2. Description of the Background Art
In recent years, high performance and small size are key factors for digital (e.g., WCDMA) mobile telephone terminals. Radio frequency power amplifiers which are used in the mobile telephone terminals so as to perform power amplification to output a high power, require small size, high efficiency, low distortion, and low noise.
A transistor included in radio frequency power amplifiers for mobile telephone terminals is often a heterojunction bipolar transistor (HBT) made of a gallium arsenide (GaAs) material which enables a high-speed operation. The HBT has a high current amplification factor β and a small third-order distortion, and therefore, is widely known as a device suitable for digital-modulation mobile telephone systems which require a highly linear operation. Particularly, a radio frequency power amplifier which handles a radio frequency signal has a multi-finger structure in which a plurality of HBTs having an emitter finger are connected in parallel, and is configured so that a radio frequency signal is input to the base of each HBT and the collector outputs of the HBTs are combined to obtain a high power output.
However, when the multi-finger structure is used to try to obtain a high power output, since the thermal conductivity of the GaAs substrate is smaller than silicon and the like, the temperature increase of the device becomes significant, depending on the output, likely leading to a deterioration in radio frequency characteristics. Particularly in the HBT, a high power output causes a temperature increase, so that a base-emitter voltage Vbe decreases, resulting in an increase in collector current. Therefore, if concentration of a current (an increase in collector current) occurs in any one of the HBTs in the multi-finger structure for some cause, a temperature increase occurs, so that further current concentration occurs in the one HBT. When such non-uniformity occurs in a current distribution, a specific HBT no longer performs a desired operation, so that a power corresponding to the plurality of HBTs connected in parallel cannot be obtained, resulting in a deterioration in radio frequency characteristics. When this phenomenon develops, the HBT may go into thermal runaway and be broken down.
Conventional radio frequency power amplifiers which solve the problem have been proposed in U.S. Pat. No. 5,608,353 (Patent Document 1), Japanese Patent Laid-Open Publication No. 2001-274636 (Patent Document 2), and the like. In the radio frequency power amplifier described in Patent Document 1, a ballast resistance is inserted into the base of each HBT, and a negative feedback is applied to the base-emitter voltage Vbe of each HBT with respect to a current increase, thereby preventing current concentration into a specific HBT to provide a uniform distribution. Thereby, breakdown due to thermal runaway and a deterioration in radio frequency characteristics can be eliminated.
FIG. 8 illustrates an exemplary circuit of a conventional radio frequency power amplifier 100 in which n HBTs (n: an integer of 2 or more) are connected in parallel. In FIG. 8, a direct-current bias voltage DC is applied via resistances R101 to R10n to the bases of transistors Q101 to Q10n. A radio frequency signal RF is input via capacitor C101 to C10n to the bases of the transistors Q101 to Q10n. All the emitters of the transistors Q101 to Q10n are grounded, and amplified signals output from the collectors of the transistors Q101 to Q10n are combined into one.
In the case of the configuration of FIG. 8, even when current concentration occurs in any of the transistors Q101 to Ql0n for some cause, a voltage drop corresponding to a base current occurs in the resistances R101 to R10n connected between the bases of the respective transistors Q101 to Q10n and the input terminal of the bias voltage DC. The voltage drop relaxes the current concentration, so that a uniform collector current flows through the transistors Q101 to Q10n, resulting in a uniform operation. Therefore, a stable operation can be achieved without breakdown due to thermal runaway and a deterioration in radio frequency characteristics.
However, when the conventional radio frequency power amplifier 100 thus configured is applied to an apparatus which transmits and receives a radio frequency signal RF, the reception of the radio frequency signal RF by the apparatus is considered to be affected as follows. For example, in a WCDMA mobile telephone system, specific codes are assigned to data by signal spectrum spread for the purpose of communication. Also in the WCDMA mobile telephone system, a FDD (Frequency Division Duplex) method is used so as to simultaneously perform transmission and reception with respect to a mobile telephone terminal.
In such a mobile telephone system, a 1950-MHz band is used for a transmission frequency, a 2140-MHz band is used for a reception frequency, a transmission output level at an antenna end of a mobile telephone terminal is a maximum of about 25 dBm (1 mW=0 dBm), and a reception input level is a minimum of about −80 dBm. In this case, if noise characteristics of a transmitted signal Tx in a reception band are not satisfactory, noise occurs in a received signal Rx, so that appropriate signal demodulation cannot be performed, eventually leading to a deterioration in speech quality (reception band noise NRx).
The reception band noise NRx of the radio frequency power amplifier can be divided into noise occurring from the device itself and noise due to InterModulation (IM). In order to reduce the reception band noise NRx of the radio frequency power amplifier, it is considerably important to particularly reduce the noise occurring due to intermodulation.
As used herein, the noise occurring due to intermodulation refers to noise which, when a transmission frequency (basic wave) is modulated with an arbitrary frequency component, appears as a distorted component at a frequency which is at a difference between these frequencies away from the basic wave. Assuming that a 1950-MHz band is used for the transmission frequency and a 2140-MHz band is used for the reception frequency, it is particularly important to handle a signal in a 190-MHz band which is a difference between these frequencies, and a signal in a 95-MHz band which is a ½ frequency of the difference frequency in view of an influence of intermodulation on the reception band noise NRx due to the transmitted signal Tx.
FIGS. 9A and 9B illustrate characteristics of a second-order intermodulation distortion (IMD2) and a third-order intermodulation distortion (IMD3) of the radio frequency power amplifier, respectively. When the transmitted signal Tx of 1950 MHz and a signal of 190 MHz are input to the radio frequency power amplifier, a noise component (IMD2) occurs in a 2140-MHz band which is at 190 MHz away from 1950 MHz (basic wave). When the transmitted signal Tx of 1950 MHz and a signal of 95 MHz are input to the radio frequency power amplifier, a noise component (IMD3) occurs in a 2140-MHz band which is at 190 MHz away from 1950 MHz (basic wave). Reception band noise characteristics deteriorate with magnitudes of IMD2 and IMD3. The magnitudes of IMD2 and IMD3 correspond to magnitudes of the transmitted signal Tx of 1950 MHz, the signal of 190 MHz, and the signal of 95 MHz. Considering that 1950 MHz corresponds to a maximum output of 25 dBm, it is clearly understood that it is important to decrease the signals of 190 MHz and 95 MHz so as to reduce IMD2 and IMD3.
FIG. 10 illustrates frequency pass characteristics of each HBT in a conventional radio frequency power amplifier comprising three HBTs. In these three HBTs, the radio frequency signal RF is input via a capacitor to the base of each HBT, and relatively large gains are obtained: a gain in the 190-MHz band (IMD2) is about −10 dB; and a gain in the 95-MHz band (IMD3) is about −20 dB. Note that the HBTs have the same frequency pass characteristics. Therefore, a gain of the radio frequency signals RF combined at an output terminal Pout is +14.8 dB. In such a case, a gain in the 190-MHz band is −5.2 dB and a gain in the 95-MHz band is −15.2 dB, i.e., these values are relatively large, so that IMD2 and IMD3 are not sufficiently reduced, resulting in high reception band noise characteristics.
FIG. 11 illustrates a relationship between the transmitted signal Tx and the received signal Rx, and the noise characteristics of the radio frequency power amplifier when a state of radio wave is poor (at an antenna end, a transmission output is maximum and a reception input is minimum). The level of the received signal Rx is considerably smaller than that of the transmitted signal Tx, so that the noise characteristics of the radio frequency power amplifier are not sufficiently reduced in the vicinity of a reception band, and have almost the same level as that of the received signal Rx. Therefore, it is difficult to identify the received signal Rx, so that a signal to be originally demodulated is not read, resulting in an increase in code error rate and a deterioration in speech quality.